Add instruction to lock/unlock `unreleased` after content assembly

Why is this change being made?

Follow-up action item from release post 13.6 retro that better prevents items being merged to master after content assembly has run and missing the release.

cc @mkarampalas for alignment on 13.7 - instructions are in the MR on how to do this and happy to clarify language is this is confusion 🙇

Author Checklist

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